1. Field of the Invention
This invention relates to spool valves and more particularly to a spool for a spool valve and a method of producing same.
2. Description of the Prior Art
The spool of a spool valve normally comprises at least one annular land which cooperates with at least one annular land on the surface of a passage in a valve body in which the spool is slidable to control flow through the valve. In order to meter the fluid flow through the valve it is known to provide notches in a side surface of said land which extend inwardly from the periphery thereof. For reasons of cost, such notches are conventionally produced using a cylindrical or parallel sided milling cutter introduced radially of the spool at two or more positions around the circumference of the spool. Such spools suffer from the disadvantage that, due to the position of the inlet and outlet ports within the valve body, the flow rate through the valve can vary with the angular position of the spool given a constant differential pressure between the inlet and outlet ports of the valve, with the sensitivity to angular position of the spool increasing with increasing flow rate. Another disadvantage is that in order to maximise the flow rate large notches must be used and that when these are produced using a cylindrical cutter a segment of spool material is left between each pair of adjacent notches which is both mechanically weak and can easily become over hardened by heat treatment. Conventionally, to overcome this, a notch diameter is selected which leaves an acceptable wall thickness between the notches at the expense of flow rate.
Advantageously, the notches should be designed so that as the spool moves from an initial position at which flow is wholly through the notches there is a gradual increase in the percentage of the circumference of the spool contained within the notches to provide a smooth transition to a spool position at which flow can occur around the full circumference of the spool. Thereafter, with increasing spool displacement, metering should remain at full circumference of the spool to reduce the susceptibility of the flow to the angular position of the spool. With the known parallel-sided notch form it is not possible to blend the notches together to give the required smooth transition without either sacrificing flow rate or producing an unacceptable shape in the remaining spool material.